A forward error correction codeword (FEC) includes an information element and a supervision element. When decoding is performed on an FEC codeword, an input information sequence of a decoder needs to be synchronized and aligned with an output information sequence of an encoder, that is, it is required that a start location for decoding by the decoder needs to be a start location of the FEC codeword. If the start location for decoding by the decoder is not the start location of the FEC codeword, the decoder cannot identify an information element and a supervision element in the FEC codeword, and decoding cannot be performed correctly.
A common method in the prior art is adopting a manner in which an FEC codeword and a frame structure are in an absolute mapping relationship, that is, for a resource block including N symbols and M frequency resources in two dimensions, a unique mapping relationship exists between a quantity of bits that can be carried in each resource block and an FEC codeword, that is, one resource block is corresponding to one FEC codeword.
In another prior art, a synchronization header or a start identifier is added to each FEC codeword at a transmit end, that is, a synchronization field or identifier is added before each FEC codeword, and is used by a receive end to perform codeword synchronization.
In a process of implementing the foregoing FEC codeword synchronization, the inventor finds that at least the following problem exists in the prior art:
If the manner in which an FEC codeword and a frame structure are in an absolute mapping relationship is adopted, because a quantity of bits carried in a resource block may be any value, flexibility of a time-frequency resource block can be met only when a size of an FEC codeword has a quite wide dynamic range. However, higher flexibility of an FEC codeword brings more trouble. Therefore, mapping is performed on a resource block only by fixing several lengths of FEC codewords, which causes relatively low utilization of the resource block.
If a synchronization header or a start identifier is added to each FEC codeword, in a case in which a length of an FEC codeword is relatively short, or when a receiver that has made a codeword synchronized does not need to keep performing codeword synchronization, this design brings relatively more unnecessary overheads to a system. Therefore, how to reduce system overheads when FEC codeword synchronization is ensured is an urgent problem to be resolved.